linux-kernel - Re: possible circular locking dependency mmap_sem/cpu_hotplug_lock.rw

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Date:   Tue, 5 Sep 2017 12:23:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Artem Savkov <asavkov@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: possible circular locking dependency
 mmap_sem/cpu_hotplug_lock.rw_sem

On Tue 05-09-17 10:19:13, Thomas Gleixner wrote:
> On Mon, 4 Sep 2017, Michal Hocko wrote:
> 
> > Thomas, Johannes,
> > could you double check my thinking here? I will repost the patch to
> > Andrew if you are OK with this.
> > > +	/*
> > > +	 * The only protection from memory hotplug vs. drain_stock races is
> > > +	 * that we always operate on local CPU stock here with IRQ disabled
> > > +	 */
> > >  	local_irq_save(flags);
> > >  
> > >  	stock = this_cpu_ptr(&memcg_stock);
> > > @@ -1807,26 +1811,27 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> > >  	if (!mutex_trylock(&percpu_charge_mutex))
> > >  		return;
> > >  	/* Notify other cpus that system-wide "drain" is running */
> > > -	get_online_cpus();
> > >  	curcpu = get_cpu();
> 
> The problem here is that this does only protect you against a CPU being
> unplugged, but not against a CPU coming online concurrently.

Yes but same as the drain_all_pages we do not have any cpu up specific
intialization so there is no specific action to race against AFAICS.

> I have no idea
> whether that might be a problem, but at least you should put a comment in
> which explains why it is not.

What about this?
---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5c70f47abb3d..ff9b0979ccc3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1810,7 +1810,12 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 	/* If someone's already draining, avoid adding running more workers. */
 	if (!mutex_trylock(&percpu_charge_mutex))
 		return;
-	/* Notify other cpus that system-wide "drain" is running */
+	/*
+	 * Notify other cpus that system-wide "drain" is running
+	 * We do not care about races with the cpu hotplug because cpu down
+	 * as well as workers from this path always operate on the local
+	 * per-cpu data. CPU up doesn't touch memcg_stock at all.
+	 */
 	curcpu = get_cpu();
 	for_each_online_cpu(cpu) {
 		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);

-- 
Michal Hocko
SUSE Labs